Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and a rotor including one or more rotor blades. The rotor blades capture kinetic energy from wind using known foil principles and transmit the kinetic energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
During operation of a wind turbine, various components of the wind turbine are subjected to various loads due to the aerodynamic wind loads acting on the blade. In particular, the rotor blades experience significant loading, and frequent changes in loading, during operation due to interaction with the wind. Changes in wind speed and direction, for example, may modify the loads experienced by the rotor blades. To reduce rotor blade loading, various methods and apparatus have been developed to enable pitching of the rotor blades during operation. Pitching generally allows the rotor blades to shed a portion of the loads experienced thereby.
The amount of power produced by a wind turbine is typically constrained by structural limitations of the individual wind turbine components. The power available from the wind is proportional to the area of the rotor, and the square of the rotor diameter. Thus, the amount of power produced at different wind speeds can be significantly higher by increasing the diameter of the rotor of the wind turbine. Such an increase in rotor size, however, also increases mechanical loads and material costs in a way that may exceed the corresponding increase in energy production. Further, though it is helpful to control power and rotor speed, thrust from the wind on the rotor truly drives many dominant fatigue loads, along with any asymmetry of that thrust. The thrust force comes from a change in pressure as the wind passes the wind turbine and slows down. The terms “thrust,” “thrust value,” “thrust parameter” or similar terms are generally used in the art to encompass a force acting on the wind turbine due to the wind and in the general direction of the wind, and may also be used to describe inputs to a control method of a value that changes in direct proportion to thrust in an operating region of interest (e.g. individual or average out-of-plane blade or flap-wise bending, tower bending, or tower top acceleration).
Recent developments in the wind power industry have led to new methods of mechanical-load-reducing controls that allow larger rotor diameters to be employed with less than proportional increases in material costs. For example, some modern wind turbines may implement drive train and tower dampers to reduce loads. In addition, modern wind turbines may utilize individual and collective blade pitch control mechanisms to reduce fatigue and extreme loads, thereby enabling higher ratios between rotor diameter and structural loads while also lowering the cost of energy.
Conventional wind turbines are designed for a rated wind speed at which maximum thrust and maximum power generation occur. At rated wind speed, the turbine controller attempts to limit estimated thrust to a control threshold value (e.g., a value of 350 kN at rated wind speed). At wind speeds higher than rated wind speed, the rotor blades are pitched to reduce thrust. Many methods are known for determining whether to pitch the rotor blades in order to reduce thrust. However, such thrust control should not unnecessarily limit power output of the wind turbine.
Accordingly, improved methods are desired for controlling wind turbine loading as a function of thrust control without prematurely or unnecessarily limiting power output.